In solar cells, a greater part of the photocarriers are generated in a surface region of an active layer and, therefore, thin-film solar cells, in which an active layer contributing to power generation is a thin film to reduce the cost of materials and production time of the active layer, have been developed.
FIG. 9 is a perspective view illustrating a structure of a prior art thin-film solar cell. In FIG. 9, reference numeral 100 designates a thin-film solar cell which generates electric power by converting incident light to electricity. In the solar cell 100, an active layer 20 several tens of microns thick including a p-n junction and contributing to power generation is disposed on a conductive substrate 50. An anti-reflection film 30 is disposed on the active layer 20 and prevents the incident light from being reflected at the surface of the active layer. An upper electrode 40 is disposed on the anti-reflection film 30 and a lower electrode 60 is disposed on the rear surface of the conductive substrate 50. The upper electrode 40 comprises a grid electrode 40a for collecting photoelectric current generated in the active layer 20 and a bus electrode 40b for concentrating the photoelectric current from the grid electrode 40a.
In this thin-film solar cell 100, since the active layer 20 contributing to power generation is as thin as several tens of microns, it cannot mechanically support itself, so that a substrate or the like for supporting the thin active layer 20 is needed. The following conditions are required of the substrate.
First, the substrate should be strong enough to automatically support the thin film and itself. Second, since the Si thin film active layer is grown on the substrate by thermal CVD or the like, the substrate should be refractory so that it can stand a process temperature of approximately 1000 C. during the thermal CVD. Third, since the substrate also serves as a lower electrode, it should be conductive. Even if the substrate is not conductive, a thin-film solar cell can be achieved. In this case, however, it is necessary to dispose a conductive film on the substrate or draw out the lower electrode from the side surface of the solar cell, resulting in a complicated structure. Fourth, since the substrate itself does not contribute to power generation but only supports the active layer, it is desirable that the substrate be formed by an inexpensive method using an inexpensive material.
However, a material for such a substrate with sufficient mechanical strength and heat resisting property is expensive. Furthermore, when the substrate should also be conductive, it is very difficult to select a material for the substrate.
Therefore, in a prior art method of producing a thin-film solar cell, after a semiconductor thin film is formed on a heat resistant substrate at a high temperature, the semiconductor thin film is removed from the heat resistant substrate and adhered to a cheap substrate. The heat resistant substrate is repeatedly used. Since actual process steps in this prior art method are complicated, a basic concept thereof will be described first with reference to FIGS. 10(a)-10(d).
As illustrated in FIG. 10(a), a layer 4 comprising sintered silicon nitride powder is formed on the surface of the heat resistant substrate 1. Then, as illustrated in FIG. 10(b), a semiconductor thin film 2 is grown on the layer 4 by thermal CVD or the like. Thereafter, the semiconductor thin film 2 is subjected to process steps, such as annealing, recrystallization, or formation of a p-n junction, as occasion requires. After the processing, a glass substrate 3 is adhered to the semiconductor thin film 2 as illustrated in FIG. 10(c).
Thereafter, the layer 4 is broken to separate the semiconductor thin film 2 from the heat resistant substrate 1 as illustrated in FIG. 10(d). Since the layer 4 comprises sintered silicon nitride powder, there is a weak bonding strength between particles. Therefore, when mechanical stress is applied to the heat resistant substrate 1 and the glass substrate 3 to separate them from each other, only the layer 4 is broken.
In this way, the semiconductor thin film 2 is removed from the heat resistant substrate 1 and adhered to the glass substrate 3, and the subsequent processing is carried out with the semiconductor thin film 2 disposed on the glass substrate 3.
The above-described method of producing a thin-film solar cell will be described in more detail according to actual process steps.
FIGS. 11(a)-11(k) are cross-sectional views illustrating process steps in a prior art method of producing a thin-film solar cell. In the figures, a layer 4 is disposed on a heat resistant substrate 1 comprising silicon. The layer 4 is formed by applying a paste, comprising silicon nitride powder, to the substrate and sintering the paste. A semiconductor thin film 2a is disposed on the silicon nitride layer 4. The semiconductor thin film 2a is formed by growing p type polycrystalline silicon on the silicon nitride layer 4 by vapor phase deposition. The polycrystalline Si thin film 2a is covered with a cap layer 5. When the thin film 2a is melted and recrystallized, the cap layer 5 prevents the melted portion of the thin film 2a from condensing and separating into islands.
Reference numeral 2 designates a p type semiconductor thin film in which the diameters of crystal grains are increased by melting and recrystallizing the polycrystalline Si thin film 2a. An n.sup.+ type region 6 is formed at the surface of the thin film 2 by diffusing a dopant like phosphorus, arsenic, or antimony into the p type semiconductor thin film 2, and a p-n junction is produced at the boundary between the n.sup.+ type region 6 and the p type semiconductor thin film 2. The thickness of the n.sup.+ type diffused region 6 is appropriately selected within a range of 100 angstroms to 1 micron. In place of the n.sup.+ type diffused region 6, an n type microcrystalline film may be used, which is obtained by depositing silicon including an impurity like phosphorus on the semiconductor thin film 2. A conductive anti-reflection film 7 is disposed on the n.sup.+ type diffused region 6. The anti-reflection film 7 reduces reflection of incident light from the semiconductor thin film 2. A grid electrode 8 for collecting photoelectric current generated in the semiconductor thin film 2 is disposed on the anti-reflection film 7.
A cover glass 3 is adhered to the surface of the semiconductor thin film 2 via a resin 9 like EVA (Ethylene Vinyl Acetate). This cover glass 3 serves as a substrate for supporting the semiconductor thin film 2 after the semiconductor thin film 2 is separated from the heat resistant substrate 1. A rear electrode 11 comprising Ag is disposed on the rear surface of the semiconductor thin film 2 and a p.sup.+ type BSF (Back Surface Field) layer 10 is disposed between the semiconductor thin film 2 and the rear electrode 11. The p.sup.+ type BSF layer 10 forms an energy barrier in the semiconductor thin film 2 in the vicinity of the rear electrode 11 and prevents photocarriers from reaching the boundary between the semiconductor thin film 2 and the rear electrode 11, whereby annihilation of photocarriers in the vicinity of the boundary is avoided.
A description is now given of the production method.
First of all, the heat resistant substrate 1 shown in FIG. 11(a) is prepared. Then, a paste comprising silicon nitride powder is applied to the heat resistant substrate 1 and sintered to form the layer 4 about 100 microns thick (FIG. 11(b)). Then, p type polycrystalline silicon is grown on the layer 4 by vapor phase deposition to form the p type polycrystalline Si thin film 2a about 30 microns thick (FIG. 11(c)). Thereafter, SiO.sub.2 is deposited on the polycrystalline Si thin film 2a to form the cap layer 5 (FIG. 11(d)). Then, zone melting and recrystallization of the polycrystalline Si thin film 2a is carried out. More specifically, a portion of the polycrystalline Si thin film 2a is melted by laser radiation or the like and this portion is moved through the polycrystalline Si thin film 2a to recrystallize the thin film 2a (FIG. 11(e)). Thereafter, the cap layer 5 is removed (FIG. 11(f)).
Then, an n type dopant like phosphorus is diffused into the surface region of the semiconductor thin film 2 to form an n.sup.+ type diffused region about several thousands of angstroms thick, whereby a p-n junction is produced in the semiconductor thin film 2. Then, an anti-reflection film 7 about several hundreds of angstroms thick is formed on the n.sup.+ type diffused region 6. Then, a grid electrode 8 comprising a lower Ti layer and an upper Ag layer is formed on the anti-reflection film 7, completing the process steps with respect to the upper side of the thin-film solar cell (FIG. 11(g)). In addition, the anti-reflection film 7 is a transparent electrode comprising ITO (Indium Tin Oxide).
Thereafter, a glass substrate 3 is adhered to the surface of the thin-film solar cell via the resin 9, such as EVA (FIG. 11(g)). This glass substrate 3 serves as a cover glass when a solar cell module is produced. This glass substrate 3 may be directly adhered to the surface of the solar cell by welding. Then, a mechanical stress is applied to the heat resistant substrate 1 and the glass substrate 3 to separate them from each other, so that the layer 4 is broken and the the semiconductor thin film 2 is separated from the heat resistant substrate 1 (FIG. 11(i)).
This separation process will be described in more detail with reference to FIGS. 12(a) and 12(b). Initially, as illustrated in FIG. 12(a), the wafer shown in FIG. 11(h) is put on a stage 70 so that an end of the wafer abuts a projection 71. Then, a wedge 72 is inserted into the silicon nitride layer 4 exposed on the side surface of the wafer to crack the layer 4. Then, as illustrated in FIG. 12(b), the glass substrate 3 is picked up with tweezers 74 while the wafer is fixed by pressing the heat resistant substrate 1 against the projection 71 using a push rod 73, whereby the semiconductor thin film 2 is separated from the heat resistant substrate 1.
Then, fragments 4a of the layer 4 remaining on the surface of the semiconductor thin film 2 are etched away using heated phosphoric acid. Then, a paste including a p type impurity like aluminum is applied to the surface of the thin film 2 and then it is sintered to diffuse the p type impurity into the thin film 2, whereby a p.sup.+ type BSF layer 10 about several microns thick is formed. Then, the sintered paste is removed, or it may be left on the BSF layer 10. Then, the rear electrode 11 comprising Ag is formed on the surface of the p.sup.+ type BSF layer 10, completing process steps with respect to the rear side of the solar cell (FIG. 11(k)).
In this way, the thin-film solar cell 120 is produced.
In the conventional production method, however, since the layer 4 comprises silicon nitride which is an insulating material, after the semiconductor thin film 2 is separated from the heat resistant substrate 1, it is necessary to remove the fragments 4a of the layer 4 remaining on the surface of the semiconductor thin film 2. In addition, since the layer 4 is obtained by sintering a paste comprising silicon nitride powder, the composition thereof is not uniform. That is, silicon nitride acts upon other materials to produce other compounds in some portions of the layer 4 while silicon nitride remains in the other portions. In this case, it is difficult to completely remove the fragments 4a remaining on the semiconductor thin film 2 by etching using heated phosphoric acid which is usually used for removing silicon nitride.
In addition, the layer 4 should satisfy conflicting requirements, i.e., it should have a mechanical strength strong enough to withstand the stress applied thereto during the formation of the semiconductor thin film 2 and, at the same time, it should be easily broken when the semiconductor thin film 2 is separated from the heat resistant substrate 1. However, the breaking stress of a material does not vary according to the direction of the stress applied. More specifically, if a breaking stress in a certain direction is large, a breaking stress in another direction is large. This holds good for the layer 4 comprising sintered silicon nitride powder, too. That is, it is very difficult for the layer 4 to decrease in strength against a stress in a direction along which the layer 4 is broken while keeping sufficient mechanical strength. Therefore, adhesion between the semiconductor thin film and the heat resistant substrate and the simplicity of separating the semiconductor thin film from the heat resistant substrate are trade-offs.
Accordingly, it is difficult to realize a favorable formation of the semiconductor thin film on the heat resistant substrate 1 and an easy separation between the semiconductor thin film and the heat resistant substrate. As a result, it has been impossible to produce a high-quality thin film solar cell with high reliability at low cost.